A jet engine nacelle is the primary structure for managing noise from a variety of jet engine functions. Traditionally, “one degree of freedom” (1DoF) honeycomb structures have offered modest noise reductions. These 1DoF structures are typically honeycomb sandwich panels that include holes through the panel skin facing the engine. These holes create a single chamber in which sound reverberates and cancels, thus creating acoustic sound absorption. However, these traditional panel structures provide no improvement in sound transmission loss (STL) across the wall of the nacelle.
More recently “two degree of freedom” (2DoF) structures were developed in an effort to improve sound absorption. These 2DoF structures use a porous septum to separate two resonating honeycomb chambers. This septum is generally a woven cloth or a perforated sheet. However, when the septum is adhered to the honeycomb chambers, the bonding process may cause adhesives and adhesive films to inadvertently cover one or more honeycomb openings thereby eliminating the ability of those covered chambers to efficiently absorb sound. Specifically, U.S. Pat. No. 5,041,323 involves a perforated septum sheet bonded between two honeycomb panels. The 323 patent discloses sintering, diffusion bonding, sprayable adhesives or adhesive films, but does not disclose any means for preventing the adhesive from covering the honeycomb openings.
U.S. Pat. No. 6,736,181 discloses a panel fabrication method that utilizes a film adhesive by locating the adhesive on a panel opposing the honeycomb openings. The 181 patent requires temporary adhesion of the film to the panel without curing the adhesive prior to locating the panel into a reticulation unit.
U.S. Pat. Nos. 7,434,659; 7,510,052; and 7,528,195 disclose inserting a cloth septum into each cell of the honeycomb to create a 2-chambered 2DoF construction. The step of locating the septum into each honeycomb cell increases fabrication time and requires specialized machinery.
Accordingly, there remains a need in the art to find improved and simplified materials that increase sound transmission loss and acoustic absorption. Further, there is a need for a panel assembly that can be easily assembled with simplified machinery and does not require insertion of a septum into each honeycomb cell and does not run the risk of film or liquid adhesives that cover the honeycomb cells. There is a further need for panel construction methods that can easily and inexpensively be tuned for a specific frequency.